Hollow stackable plastic products

ABSTRACT

A molded thin-walled hollow, stackable plastic product, having an open top, a closed bottom and side walls; wherein the side walls include a series of circumferential sections joined by circumferential strips, with the upper of two adjacent wall sections having a greater diameter than the lower adjacent wall section to thereby define a circumferential ridge at the bottom of each upper wall section. The circumferential strip joining a pair of the wall sections is a circumferential isthmus having an isthmus distance, which is the minimum distance between the mold parts defining the isthmus, that is significantly shorter than the predominant thickness of the adjacent wall sections. The isthmuses, which are aligned at an acute angle with respect to each of the joined wall sections, improve the rigidity of the molded thin-walled hollow product. The wall sections include a plurality of spaced protruding ridges extending between adjacent circumferential strips. When the molded product is a hand-held vending cup the combination of the circumferential ridges and the spaced protruding ridges prevent burning of the fingers.

This is a continuation of co-pending application Ser. No. 643,844 filedon Aug. 23, 1984, now abandoned which is a div. of Ser. No. 424,493filed Sept. 27, 1982, now U.S. Pat. No. 4,467,994

FIElD OF THE INVENTION

This invention generally relates to hollow stackable plastic productsproduced by the injection molding and is particularly directed toproducts produced by centering the core of the mold. The core is themold part which forms the interior of a concave or hollow moldedproduct.

CROSS REFERENCE TO RELATED APPLICATION

The invention is cross referenced to U.S. patent application Ser. No.230,302, now U.S. Pat. No. 4,381,275 entitled STABILIZED CORE INJECTIONMOLDING OF PLASTIC and United Kingdom Patent Application No. 8,221,909entitled STABILIZED CORE INJECTION MOLDING, both applications which aredifferent are by the inventor of the present application.

DISCUSSION OF PRIOR ART

Heretofore, it has been a great problem to center the cores of moldsused for injection molding of hollow stackable products of plastic withthin walls such as vending cups, yogurt containers, margerine containersand flower pots. In order to injection mold such products a very largeinjection pressure is necessary which causes the core part of the moldto move in relation to the cavity part of the mold, resulting in moldedproducts which do not have even wall thickness.

Until now no practical solution has been found to the problem andtherefore such products are generally produced with a wall thicknesswhich is larger than necessary for the purposes of the product. With thelarger wall thickness lower injection pressure is sufficient for fillingthe mold, and therefore less strain is put on the core part of the mold,with resulting more evenly distributed wall thickness.

Many unsuccessful attempts have been made to solve the problem of corecentering, for example U.S. Pat. No. 4,264,245 to Hingley; and U.S. Pat.No. 3,375,554 to Blumer, which is the closest prior art to the presentinvention, a description of the mold disclosed by Blumer and the reasonthat it is not successful is illustrated in the drawings and theirdescription.

SUMMARY OF THE INVENTION

The present invention is a thin-walled, hollow, stackable plasticproduct produced by injection molding with a mold, comprising a corepart and a cavity part where the relative reciprocal wall thickness ofthe molded product may be equalized by centering the core part in thecavity part during injection.

The core part and the cavity part each have an axially extendingcircumferential recess in a circumferential shoulder. Each such recesslimits an axially extending annular projection (ridge). Correspondingsuch annular projections located on the sides of the core part and thecavity part respectively in combination define a core centeringcircumferential throttle. The minimum distance from the ridge of thecore part to the corresponding ridge of the cavity part is called thethrottle distance, when the core part is centered in the cavity part thethrottle distance of the invention is significantly shorter than theadjacent wall thickness. The circumferential throttle defines acircumferential strip in the form of a circumferential isthmus in themolded product, whereby the throttle distance corresponds to an isthmusdistance in the molded product.

The molded thin-walled hollow, stackable plastic product of the presentinvention has an open top, a closed bottom and side walls; wherein theside walls include a series of circumferential sections joined bycircumferential strips, with the upper of two adjacent wall sectionshaving a greater diameter than the lower adjacent wall section tothereby define a circumferential ridge at the bottom of each upper wallsection. The circumferential strip joining a pair of the wall sectionsis a circumferential isthmus having an isthmus distance, which is theminimum distance between the mold parts defining the isthmus, that issignificantly shorter than the predominant thickness of the adjacentwall sections. The isthmuses, which are aligned at an acute angle withrespect to each of the joined wall sections, improve the rigidity of themolded thin-walled hollow product. The wall sections include a pluralityof spaced protruding ridges extending between adjacent circumferentialstrips. When the molded product is a hand-held vending cup thecombination of the circumferential ridges and the spaced protrudingridges prevent burning of the fingers. For purposes of stacking themolded hollow products the angle of the pitch between the adjacentcircumferential ridges is larger than are tangent of the predominantadjacent section wall thickness divided by the distance between theadjacent horizontal ridges. The circumferential ridges on the side ofthe core and cavity aprt of the mold shape the circumferential isthmuseson the hollow stackable molded product. Each circumferential isthmus isaligned at an acute angle with respect to each of the wall sectionsjoined by the isthmus. These isthmuses help to make the hollow productrigid. When the hollow product is, for example, a hand-held vending cupfor hot drinks, the combination of the spaced protruding ridges and thecircumferential ridges provide a reduced area of contact between cup andfingers, resulting in less heat transfer and prevent possible burning ofthe fingers. There may conveniently be about ten to twentycircumferential strips on the molded product, not all of these ridgesneed to be isthmuses with an isthmus distance which is signifcantlyshorter than the predominant adjacent wall thickness. In one embodimentonly every other circumferential strip is a circumferential isthmus. Thecircumferential isthmuses which are nearer the bottom of the hollowproduct have shorter isthmus distances than the isthmuses which arecloser to the open end of the hollow product.

In order to ease the filling of the cavity for injection molding of thethin-walled hollow product, a number of feed channels may be provided inthe core and/or cavity part of the mold leading in a direction from theapex towards the open end of the hollow product. In order to get thebest core centering, it is important that the throttle distance,particularly of the throttles near the apex end of the hollow product,is small in relation to the adjacent wall, thickness where moltenplastic is flowing during the filling. During the filling step, a largeproportion of the injected plastic will flow in the above mentioned feedchannels, and therefore the throttle distance in relation to the wallthickness of the feed channels is particularly small, thereby greatlyimproving the core centering process. The feed channels define theprotruding spaced ridges in the molded thin-walled product.

Further objects and advantages of the invention will become apparentfrom a consideration of the drawings and subsequent description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a sectional view of a prior art mold for injectionmolding of hollow stackable plastic products.

FIG. 1B illustrates a perspective view of the cavity part of the mold ofFIG. 1A.

FIG. 2A illustrates a sectional view of a mold according to a preferredembodiment of the invention.

FIG. 2B illustrates an enlarged sectional view of a throttle located inthe circled area marked B of FIG. 2A.

FIG. 2C illustrates a perspective view of a hollow stackable plasticproduct as produced in the mold of FIG. 2A.

FIG. 3 shows a partial sectional view of two hollow stackable plasticproducts in their stacked position as produced by a mold of a preferredembodiment of the invention.

FIG. 4 shows a partial sectional view of two hollow stackable plasticproducts in their stacked position as produced by a mold of anotherpreferred embodiment of the invention.

DESCRIPTION OF THE ILLUSTRATED PRIOR ART

FIG. 1A shows a prior art mold comprising a core part 101 and a cavitypart 102 for injecting molding a thin-walled, hollow, stackable plasticproduct. The cavity part 102 comprises a circumferential protuberance103 which projects into a circumferential depression 104 of the apex ofthe core part 101. The protuberance 103 and the depression 104 incombination provide a circumferential throttle 112 with a throttledistance 105a and 105b which at an average corresponds to the wallthickness of the molded product to be produced. The mold 101, 102comprises a runner 106 and a gate 107 which connects the runner 106 withthe cavity 108. A partly filled product 109 is shown in the cavity 108.Near the open end of the cavity part 102 is a conical flange 110 whichcenters the core part 101 with the cavity part 102 of the mold when themold is closed, and before plastic is injected.

FIG. 1B shows a perspective view of the cavity 108.

The operation of the prior art embodiment depicted in FIGS. 1A and 1B isas follows: molten plastic enters from the runner 106 through the gate107 into the cavity 108. The molten plastic will from practicalexperience flow further down the side of the core than the other, inthis case the side down which the molten plastic has flown the furthestis depicted as the right side. The pressure of the molten plastic on theright side of the core part 101 causes a force on the core part 101 in aleft going direction which is greater than the force in a right goingdirection caused by the pressure of the molten plastic on the left sideof the core part 101, and therefore the core is forced to the left, asshown by the arrow 113. Both the forces and movement as described aboveare in relation to the cavity part 102. The two opposing forces areproportional to the product of the pressure and the projected area; bothaverage pressure and the area is greater on the right side of the corepart 101, and therefore the force is also greater on the right side ofthe core. In the absence of the circumferential throttle 112, most ofthe molten plastic would continue to flow down the right side of thecavity 108, thereby further moving the core part 101 to the left, in avicious circle of off-centering the core part 101 in the cavity part 102of the mold. But in the case depicted in FIG. 1A, the action of the corepart 101 moving to the left will cause the right side of thecircumferential throttle 112 to close up and the left side of thethrottle 112 to open up, resulting in the flow down the right side ofthe cavity 108 being impeded and the flow down the left side of thecavity 108 being enhanced, thereby to a certain extent breaking thevicious circle. Unfortunately, practical experience shows that the abovedescribed prior art system is unsuccessful. The reason will becomeapparent from a description of the operation of the perspective view ofthe cavity in FIG. 1B. The flow of molten plastic to the right and tothe left has already been described; we shall not look at the flow ofmolten plastic which flows in a direction towards the observer.

The molten plastic flows from the gate 107 to the circumferentialthrottle 112. At the right the throttle 112 is quite closed up and atthe left the throttle 112 is quite opened up, but in a direction towardsthe observer the throttle 112 is at a wall thickness which correspondsto that of the molded product to be produced. The molten plastictherefore flows through the throttle 112 and continues in the directionof greater cavity thickness. Down the left side of the cavity 108 thethickness is decreased due to the core part 101 moving to the left, downthe front side of the cavity 108 the thickness is that of the product tobe produced, but down the right side of the cavity thickness isincreased due to the core part 101 moving to the left; the moltenplastic therefore moves down in a right direction as depicted by thearrows 111. Unfortunately, this flow bypasses the closed up side of thecircumferential throttle 105B, and the advantages of the core centeringprocess are very small indeed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2A shows a mold of a preferred embodiment of the invention. Themold comprises a core part 201 and a cavity part 202 for injectionmolding a thin-walled, hollow, stackable plastic product. The side ofthe cavity part 202 comprises several circumferential ridges 203 and theside of the core part 201 comprises an equal number of circumferentialridges 204. Each ridge 203 of the cavity part 202 corresponds to a ridge204 of the core part 201. Each corresponding pair of circumferentialridges provides a self-centering circumferential throttle 212 for theinjection of plastic. The minimum distance from the ridge of the corepart 204 to the corresponding ridge of the cavity part is called thethrottle distance 205, seen best on FIG. 2B; the throttle distance 205is significantly shorter than the adjacent wall thickness 213. The mold201, 202 also comprise a runner 206 and a gate 207 which connects therunner 206 with the cavity 208. Near the open end of the cavity part 218is a conical flange 210 which centers the core part 201 with the cavitypart 202 of the mold, when the mold is closed and before plastic isinjected. FIG. 2C shows a molded product of the mold of FIG. 2A, inorder to ease the filling of the cavity 208 a number of feed channels214 are provided in the cavity part of the mold, leading in a directionfrom the apex towards the open end of the molded product.

The operation of the preferred embodiment depicted in FIGS. 2A, 2B and2C is as follows: molten plastic enters from the runner 206 through thegate 207 into the cavity 208. The molten plastic will from practicalexperience flow further down one side of the core part 201 than theother, for example the right side, thereby forming the core slightly tothe left, thereby slightly closing the circumferential throttle 212 onthe right side, and slightly opening the circumferential throttle 212 onthe left side.

Since the throttle distance 205 is significantly shorter than theadjacent wall thickness 213, and since the circumferential throttledistance is shorter on the right side than on the left side of thecavity 208, the molten plastic will have a tendency to fill mainly theleft side of the cavity 208. Although 100% core centering is nowachieved at this stage, and the flow of molten plastic is still fasteron the right side of the core 201 than the left side of the core 201,the flowing molten plastic will soon meet a new circumferential throttle212 which is slightly more closed on the right side than the left sideso that the molten plastic will have an additional tendency to fill upmainly the left side of the cavity 208. The next circumferentialthrottle 212 will have the same tendency and so on until almost 100%core centering is achieved.

The four most important factors that the mold used in making the productof the invention is successful in relation to the prior art are: (a)that the throttle distance is significantly shorter than the adjacentwall thickness of the molded product, particularly in thecircumferential throttles which are nearest to the gate 207; (b) thatbecause of the feed channels 214, the relation of the throttle distanceto the adjacent wall thickness is further decreased; (c) the distancefrom the gate to each circumferential throttle, which is positioned onthe side of the core and cavity parts respectively, is greater than inthe prior art, making shortcuts to the flow of the molten plastic moredifficult; and (d) there are a number of circumferential throttlesinstead of only one.

FIG. 3 shows a partial sectional view of two thin-walled, hollow,stackable plastic products in the stacked position, molded by a mold ofthe invention of a slightly modified embodiment than that depicted inFIGS. 2A, 2B and 2C. Shown is an upper product 301 and a lower product302 separated by the stacking height 308. Shown is also a firstcircumferential isthmus 303 and a second circumferential isthmus 304 ofthe upper product 301. The isthmus distance 309 which is the minimumdistance from the ridge of the core part to the corresponding ridge ofthe cavity part, of the production mold of the product, is significantlyshorter than the adjacent wall thickness 311. The angle of the pitch 305from the predominant isthmus 303 to the second isthmus 304 is largerthan arcus tangent 306 of the basic horizontally measured predominantwall thickness 311 between the isthmuses 303 and 304 divided by thestacking height 308 of the molded product.

Between the isthmuses are a number of protruding ridges 307, 312 spacedas shown in FIG. 2C and leading from isthmus 304, 303, etc. A part ofthe spaced ridges 307 project towards the center axis of the moldedproduct 301, 302 and another part of the spaced ridges 312 projects awayfrom the center axis of the molded product 301,302. The molded product301, 302 and therefore also its production mold comprise a number ofcircumferential ridges which are separated by a distance which is equalto the stacking height 308 of the molded product 301, 302.

FIG. 4 shows a partial sectional view of two thin-walled, hollow,stackable plstic products in their stacked position molded by a mold ofthe invention of a slightly modified embodiment than that depicted inFIGS. 2 or 3. Shown is an upper product 401 and a lower product 402separated by the stacking height 408. Shown is also a firstcircumferential isthmus 403 and a second circumferential isthmus 404 ofthe upper product 401. The isthmus distance 409, which is the minimumdistance from the ridge of the core part to the corresponding ridge ofthe cavity part, of the production mold of the product, is significantlyshorter than the predominant adjacent wall thickness 411. The angle ofthe pitch 405 from the first isthmus 403 to the second isthmus 404 islarger than arcus tangent 406 of the basic horizontally measuredpredominant wall thickness 411, between the isthmuses 403 and 404divided by the stacking height 408 of the molded product. Between theisthmuses are a number of protruding ridges spaced as shown in FIG. 2Cand 407, 412 spaced ridges leading from isthmus to isthmus 404, 403,etc. A part of the spaced ridges 407 project towards the center axis ofthe molded product 401, 402 and another part of the spaced ridges 412projects away from the center axis of the molded product 401, 402. Themolded product 401, 402 and therefore also its production mold comprisea number of circumferential ridges which are separated by a distancewhich is approximately equal to two times the stacking height 408 of themolded product 401, 402.

While the above description contains many specificities, these shouldnot be construed as limitations on the scope of the invention, butrather as an exemplification of one preferred embodiment thereof. Manyother variations are possible, for example molds which have no flowchannels or molds for producing products with a square circumference, orproducts with rounded cross-sectionally shaped sides. Accordingly, thescope of the invention should be determined not only by the embodimentsillustrated, but by the appended claims and their legal equivalents.

I claim:
 1. A molded thin-walled hollow, .[.stackable.]. plasticproduct, having an open top, a closed bottom and side walls;wherein theside walls include .[.a series of circumferential strips,.]. .Iadd.atleast one perimetric strip joining an upper side wall section and alower side wall section, .Iaddend.with the upper .[.of two adjacent.].wall .[.sections.]. .Iadd.section .Iaddend.having a minimum.[.diameter.]. .Iadd.cross-sectional area .Iaddend.greater than amaximum .[.diameter.]. .Iadd.cross-sectional area .Iaddend.of the.[.lowe adjacent.]. .Iadd.lower .Iaddend.side wall section to therebydefine a .[.circumferential.]. .Iadd.perimetric .Iaddend.ridge at thebottom of .[.each.]. .Iadd.the .Iaddend.upper wall section; wherein the.[.circumferential.]. .Iadd.perimetric .Iaddend.strip joining .[.a pairof.]. said upper and lower wall sections is a .[.circumferential.]..Iadd.perimetric .Iaddend.isthmus having an isthmus distance which isthe minimum thickness of said strip defining the isthmus, that issignificantly shorter than the predominant thickness of the adjacentwall sections; and wherein said isthmus is aligned at an acute anglewith respect to each of the wall sections joined by said isthmus, withthe juncture of the isthmus and the upper wall section being closer tothe bottom of the product than the juncture of the isthmus and the lowerwall section.
 2. A product according to claim 1, wherein .Iadd.the sidewalls include a series of said perimetric strips joining a series ofside wall sections and .Iaddend.the wall sections include a plurality ofspaced protruding ridges extending between adjacent.[.circumferential.]. .Iadd.perimetric .Iaddend.strips.
 3. A productaccording to claim 2, wherein the spaced protruding ridges radiallyprotrude from wall section in one direction and radially protrude from awall section adjacent thereto in the opposite direction.
 4. A productaccording to claim 1, wherein the side .[.wall includes.]. .Iadd.wallsinclude .Iaddend.a plurality of said .[.circumferential.]..Iadd.perimetric .Iaddend.strips that are said .[.circumferential.]..Iadd.perimetric .Iaddend.isthmuses and .[.join said circumferential.]..Iadd.wherein each strip joins a pair of adjacent said .Iaddend.sidewall sections.
 5. A product according to claim 4, wherein .Iadd.the sidewalls further include a plurality of perimetric strips that join sidewall sections, that are not perimetric isthmuses, and that.Iaddend.alternate .[.circumferential.]. .Iadd.with said perimetric.Iaddend.strips .Iadd.that .Iaddend.are said .[.circumferential.]..Iadd.perimetric .Iaddend.isthmuses.
 6. A product according to claim 4,wherein the wall sections include a plurality of spaced protrudingridges extending between adjacent .[.circumferential.]. .Iadd.perimetric.Iaddend.strips.
 7. A product according to claim 4, wherein the isthmusdistance is shorter in the .[.circumferential.]. .Iadd.perimetric.Iaddend.isthmuses that are nearer to the bottom.